The evolution of wireless communications has resulted in a proliferation of networks of different technologies and corresponding different air interfaces. As a result, during the course of a single call, a wireless user equipment (UE) may roam among multiple radio access networks (RANs), wherein each such RAN implements a different technology than the other RANs of the multiple RANs, for example, a second generation (2G) circuit switched RAN, such as a GSM (Global System for Mobile communications) network providing primarily circuit voice service, and a packet data RAN, such as a later generation 3GPP LTE (Third Generation Partnership Project Long Term Evolution) network, a 3GPP2 UMB (Third Generation Partnership Project 2 Ultra Mobile Broadband) network, a WiMAX network, or a Wireless Fidelity network based on IEEE 802 standards, providing packet switched Voice over Internet Protocol (VoIP) and other data related services.
As the UE roams among the circuit switched RAN and the packet data RAN providing packet data services, it may be beneficial to system performance to handoff the UE between the circuit switched RAN and the packet data RAN. For example, the channel conditions associated with the latter RAN may be more favorable than the channel conditions associated with the former RAN due to such factors as fading, adjacent and co-channel interference, and available power at a serving base station (BS) or radio access network (RAN). By way of another example, an operator of both a legacy circuit network and a packet data network may desire to move the UE from one such network to the other network for purposes of system load balancing. A handoff of a UE from a 2G network to a 3GPP LTE, 3GPP2 UMB, or WiMAX network is made additionally complex when the call involving the UE is currently anchored, and is desired to remain anchored, in an Internet Protocol Multimedia Subsystem (IMS). As is known in the art, IMS is supposed to be agnostic with respect to the underlying transport layer over which IMS data is conveyed.
The 3GPP LTE, 3GPP2 UMB, and WiMAX Inter RAN technology handover standards are in their infancy. As a result, no method has yet to be defined for handing off a voice session of a UE between a legacy circuit switched network, such as a GSM network, and a 3GPP LTE, 3GPP UMB, or WiMAX network. Therefore, a need exists for a method and apparatus for implementing such a handoff.